1. Field of the Invention
This invention relates to a fabric composite of three or more layers of structural fibers, wherein at least one of the layers is biased. The fabric is further characterized in that the layers are comprised of parallel fibers but there are no "holding" or "secondary" stitches in the horizontal direction, the entire composite being maintained by stitching in the vertical direction only. The invention also relates to the process and apparatus for making this fabric composite.
2. Background of the Prior Art
Structural fabrics have a wide variety of industrial applications wherever high strength is required, but weight must be kept to a minimum. In particular, the aerospace, marine and automobile industries frequently employ structural fabric composites comprised of many layers of structural fibers saturated with a cross-linked and hardened resin as high strength materials. Structural fibers are intended to refer generally to fibers referred to as fiberglass, E-glass, S-glass, boron fibers, carbon fibers and related fibers, which can be characterized as having extremely high Young's modulus. However, fibers of lower modulus but high strength application, such as nylon, may be structural. In general, structural fibers should be distinguished from common household and apparel fabric fibers, where strength is not critical. The layers of these composites are usually biased in directions to maximize the strength of the overall products, frequently in the directions of strongest applied tension or strain.
By biased, it is intended to mean that the structural fibers of any particular layer are substantially oriented at an angle of other than 0.degree. or 90.degree. to the major axes as the fabric composite (i.e., longitudinal and lateral centerlines).
It has long been known that woven fibers are generally inappropriate for extremely high strength requirements, as the fibers themselves, and the stress applied, tend to create weak or break points where the fibers overlap in the weaving, destroying the integrity of the product and rendering the fabric relatively useless.
Accordingly, in order to achieve composites of three or more layers, which are not interwoven, it has been necessary to produce individual layers of parallel structural fibers maintained in that parallel array by "holding" or secondary fibers or layers held in place by resin, transport those individual layers to the molding site, and then "lay-up" the layers, manually rotating succeeding layers in the desired direction or bias, and thereafter saturating the produced "lay-up" with the resin and appropriately thereafter molding the layers into a single composite.
The above-described process has a number of obvious drawbacks. One is the necessity to produce individual, or "uni" layers at the textile manufacturing plant, and thereafter go through the ardous hand labor task of correctly orienting each individual layer at the molding site, which may be many miles distant from the original textile plant.
Additionally, it has been discovered that these type of "lay-up" composites or laminates, when subjected to constant high stress, for example, as in an airplane wing surface or edge, have a tendency to develop cracks or gaps between the layers of fabric, where there is only the resin to hold the fabric together. Once a flaw does appear, it quickly spreads between the layers, rapidly producing complete failure of the composite. At the same time, these lay-ups exhibit extremely low resistance to shearing forces, applied across the laminate, as there is nothing but the resin to hold the layers in vertical array. Once again, a small flaw rapidly results in complete failure of the composite.
There are some methods known to produce non-woven fabrics of more than one layer, wherein at least one of the layers is biased at an angle other than 0 or 90.degree.. One exemplary process is disclosed in Japanese Patent No. 45-33874, Oct. 30, 1970. A similar process is described in U.S. Pat. No. 2,890,579, to Mauersberger. Essentially, these processes consist of directing fibers through a rapidly oscillating weft lay down carriage, which oscillates between two advancing rows of hooks which engage the fibrous strands, and advances the strands, in parallel array into a stitching machine. However, at most, these processes can produce 2-layer fabrics and accordingly do not completely overcome the aforementioned disadvantages. Additionally, these processes are necessarily limited to forming fabrics wherein the orientation of the fibers of one layer is necessarily the opposite of the orientation of the fibers of the opposing layer, due to the oscillation of the lay down carriage.
An alternative method for making multi-layer composites of more than 2 layers, wherein the layers may each by biased individually, is disclosed in U.S. patent application Ser. No. 210,852 filed Nov. 26, 1980. That process consists of directing formed "uni" layers as described above through nip rollers oriented, with respect to a stitching machine, at an angle thereto, so that the fibers "slide" or slip across, resulting in a bias to the fabric equal to the angle of the nip rollers. However, this process has the drawback of including in the final composite the horizontal "holding" or "secondary" yarns which maintain the fibers in parallel array prior to and during biasing. These same secondary fibers add no strength to the final composite, as they exist only within the layers of parallel fibers, and are, in any event, generally not as strong as the structural fibers of the individual layers. At the same time, however, they add substantial weight to the overall laminate, sometimes making up as much to 5 to 7% of the total weight of the fabric. If it were possible to eliminate these horizontal threads, without jeopardizing the parallel array of the structural fibers in each layer, this weight reduction would have substantial impact, particularly on fuel efficiency, in light of the industries in which these composites are employed. Furthermore, this process includes 2 distinct discontinuous steps--1, formation of the uni-layer; 2, vertical stitching.
Accordingly, it is one object of this invention to provide a fabric comprised of three layers of parallel structural fibers, wherein at least one of the layers is biased, the layers being maintained by vertical stitching only, with no horizontal holding threads being present in the composite.
It is another object of this invention to provide a continuous process and apparatus whereby the abovedescribed fabric may be made.
It is yet another object of this invention to provide a fabric, and an apparatus and process for its manufacture, which may suitably be saturated with a resin and yet, upon curing, exhibit substantial resistance to inter-layer crack propagation and shear forces.
These and other objects that will become apparent may be better understood by reference to the detailed description provided below.